Recent years have seen widespread use of so-called distribution-type power supply devices such as photovoltaic power generating devices, fuel cells, storage batteries etc. These power supply devices are capable of converting direct current power to alternating current power through power control devices having an inverter, and supplying power as current sources interconnected with commercial use systems.
In addition, some of these power supply devices have independent operation functions for operating as a voltage source similarly to an Uninterruptible Power Supply (UPS). The power supply devices having such independent operation function is capable of supplying power independently from any commercial use system even when power supplied from the commercial use system stops due to a blackout or an accident.